1. Field of the Invention
The invention relates to the field of converting RGB signals into a composite video signal, and its use in video overlays.
2. Prior Art.
Some commercially available computers, particularly personal computers, provide circuitry which permits the merger of a composite video signal (e.g., NTSC signal, more specifically Proposed Standard E1A RS-170A) with a computer generated video graphics display, typically red, green, blue (RGB) signals. The RGB signals may represent a video overlay such as text intended to be displayed with the composite video signal. In a typical application an NTSC signal from a broadcasting station, video disk or tape, or other source of a composite video signal is merged with the signal representing the video overlay to provide, for example, titles or subtitles over a background image represented by the composite signal.
Often, in the prior art, the signal representing the overlay image is stored in a frame buffer. The portion of this overlay image in which the background image from the composite signal is to show through is assigned a "key color" in the buffer which is distinct from any other color in the overlay image. The buffer is then scanned out synchronously with the composite video signal and the contents of the buffer are compared, pixel by pixel, with the "key color". When the comparator indicates that the portion of the buffer being scanned out contains the "key color", a switch selects the composite video signal to be the output signal. On the other hand, when other than the "key color" is scanned in the buffer (indicating that the overlay image is being scanned) a signal from the comparator (key or keying signal) causes the switch to select the contents of the buffer. This arrangement permits the overlay image to be shown in all colors which the buffer can store, except for the "key color".
The frame buffer stores the overlay signal in digital form (e.g., RGB or an index to an RGB lookup table). The RGB signal is converted to a composite overlay signal before being coupled to the switch, thus, the switch selects between first and second analog signals. In another prior art arrangement, the composite video signal is converted to an RGB signal and the switch selects between first and second RGB signals.
Commercially available integrated circuits are used to perform the above described functions, such as MC 1378 and TDA 3301, both manufactured by Motorola Semiconductor, Inc. Phase locked loops are used to synchronize the scanning of the frame buffer with the composite video signal.
In many applications it is desirable to provide both merged RGB signals and a merged composite video signal. This allows a user to record the composite video signal while monitoring the merged images on an RGB monitor. One prior art circuit for providing both output signals first converts the composite video into an RGB signal and then merges two RGB signals. The results of this merger provide first output RGB signals. A second composite output signal is provided by converting the merged RGB signal into a composite video signal. This arrangement provides a relatively poor second video output signal that results from the second conversion. Note that there is, in effect, a "serial" double conversion, and the second conversion provides relatively poor video because there is unavoidable signal distortion from each conversion step. As will be seen with the present invention a double merging (not a double, serial conversion) is used to solve this problem and thereby providing both high quality RGB and composite signal outputs.
Video artifacts are known to occur where computer generated images are displayed on composite video monitors. These artifacts are caused in part by high frequency video signals "chroma crosstalk" that occur at the color reference frequency of 3.58 MHz for NTSC signals. There are numerous filters used that attempt to remove these artifacts. Often, a 3.58 MHz notch filter is used to prefilter the luminance component of an NTSC signal. Applicant believes that lowpass filters may be used in the prior art to remove all chroma signals, for example, above 3.0 MHz. This eliminates the artifacts but at the cost of destroying some of the image sharpness.
It has been found that when the frequency of the dot clock associated with RGB signals is not a harmonic of the color reference frequency, color artifacts appear when the signal is converted to composite video despite the presence of a 3.58 MHz notch filter. The present invention solves this problem by using additional notch filters including notch filters to filter the keying signal.